(a) Field of the Invention
The present invention relates to a cathode ray tube (CRT) and more particularly, to a cathode ray tube capable of effectively deflecting electron beams and having increased strength against external stress.
(b) Description of the Related Art
A CRT is a device for displaying images on a screen by vertically and horizontally deflecting electron beams generated from an electron gun and landing the deflected electron beams onto the phosphor layers formed on the screen. The deflection of the electron beam is controlled by a deflection yoke which is mounted on an exterior surface of a funnel of the CRT and forms vertical and horizontal magnetic fields. CRTs are generally employed for color televisions(TVs), monitors and high definition televisions(HDTV). And with the increasing use of the CRTs, there is a need to reduce the length of the CRT for increasing the brightness of the displayed image and for reducing the size of the final products, such as TVs, monitors and HDTVs.
In a CRT with reduced length, the electron beams should be deflected with wider-angles, and the deflection frequency and current supplied to the deflection yoke should be increased for the wider-angle deflections of the electron beams. As the deflection frequency and current increase, the deflection magnetic fields tend to leak to the outside of the cathode ray tube and the power consumption increases.
In order to decrease the magnetic field leakage, a compensation coil is generally mounted with the deflection yoke. When, however, the compensation coil is employed, the power consumption of the cathode ray tube further increases. Alternatively, in order to decrease the deflection power and the magnetic field leakage, it is conventionally preferable to decrease the neck diameter of the cathode ray tube and the outer diameter of the funnel near the neck side on which the deflection yoke is mounted, so that the deflection field effectively acts on the electron beams. When the neck diameter simply decreases, there are disadvantages that the resolution of the image deteriorates due to the reduced diameter of the electron gun, and the outer electron beams are likely to bombard the inner wall of the funnel, and thus not properly land on the phosphor layer of the screen.
In order to solve these problems, U.S. Pat. No. 3,731,129 discloses a funnel having a wider peripheral portion sealed to the periphery of the panel, and a deflection portion whose cross-sectional configuration gradually varies from a shape, substantially similar to that of the rectangular image produced on the panel, to a circular shape. Thereby, the vertical and horizontal coils of the deflection yoke are closely located to the passage of the electron beams, and deflect the electron beams with reduced deflection power and without the electron beams bombarding the inner wall of the funnel.
However, in U.S. Pat. No. 3,731,129, the configuration of interior surface of the funnel was not considered in designing the funnel having the rectangular cross section. Therefore, the electron beams are not effectively deflected, and the strength of the funnel against external pressure is not satisfactory.
To overcame the shortcoming, Japanese Laid-Open patent 10-154472 discloses the funnel of a cathode ray tube having a cross-section shown in FIG. 8. As shown in FIG. 8, the funnel yoke part 22 has a maximum diameter along a direction other than the horizontal axis(X) and the vertical axis(Y), for example, a rectangular section. In FIG. 8, the Pi(.theta.) represents a point at which the interior surface of the yoke part 22 and a linear line drawn from the tube axis with an angle .theta. are met. The Piv(.theta.) and Pih(.theta.) represent distances from the point Pi(.theta.) to the horizontal axis(X) and the vertical axis(Y), respectively. As shown in FIG. 8, the interior surface of the yoke part 22 is designed so that the Piv(.theta.) and Pih(.theta.) are functions of .theta. which non-linearly increase or decrease. Therefore, the Piv(.theta.) and Pih(.theta.) change with at least one maximum value, and therefore the interior surface of the yoke part 22 can be convexed to the tube axis. However, the Japanese Laid-Open patent 10-154472 designs the interior surface of the yoke without considering the strength of the cathode ray tube against the external pressure and the trajectories of the electron beams. Therefore, it is required to develop the optimum configuration of the interior surface for increasing the strength of the cathode ray tube and effective deflection of the electron beams.